The invention relates to color image processing, and more specifically, to methods and systems for color adjustment in color image processing.
In color-television systems, a chrominance signal is typically represented by two color signals, I and Q (or U and V), wherein I and Q color signals are Cartesian coordinates of the chrominance signal and are linear transformation of the U and V color signals. I and Q signals represent the weighting of x and y components of the chrominance signal in a Cartesian coordinate system. Whether a chrominance signal is encoded into I and Q signals or U and V signals depends on the standard a video system adopts. In the NTSC standard, I and Q color signals are transmitted simultaneously as quadrature-modulated waves using a single chrominance subcarrier whose phase and amplitude are modulated. I and Q color signals are separated from the chrominance signal by demodulating with the chrominance subcarrier, and subsequent color image processing is performed on the two color signals. I and Q color signals precisely specify the location of the respective picture element in the color plane. Additionally, a luminance signal is fed to a separate processing stage and subsequently combined with the two color signals in a color matrix to generate the value for the red (R), green (G), and blue (B) signals.
The chrominance signal can also be represented by polar coordinates comprising a magnitude signal and an angle signal. The angle signal carries the hue information, and the magnitude signal carries the saturation information of the chrominance signal.
Color adjustment is typically required in color image processing, especially for preferred colors such as flesh-tone, grass green, and sky blue. In U.S. Pat. No. 4,544,944, Chin describes a circuitry performing auto-flesh correction by operating on color mixture signals. The received color mixture signals are demodulated to produce the instantaneous magnitude V_mag and angle V_ang. The signal carrying the demodulated angle V_ang is applied to the address input of a ROM. The ROM is programmed to produce sines and cosines of color corrected angles if the applied angle corresponds to flesh tones. The sine and cosine values from the ROM are multiplied by the demodulated chrominance magnitude. Entries stored in the ROM comprise sinusoidal values, for example, sin 45° and cos 30°, in fixed-point representation, and these values must be rounded before storage. As a result, the corresponding output of the ROM will have errors even if the color corrected angle equals the applied angle when adjustment is not required. The error introduced by the final multiplication operation will be significant for large magnitudes V_mag.
Methods for adjusting both chrominance and luminance color signals, are also proposed. Gallagher in U.S. Pat. No. 6,438,264, for example, teaches a method of compensating the color saturation signal as well as modifying the luminance signal for the application of a tone scale function. The color saturation signal for each pixel is adjusted based on a calculated local slope of the tone scale function.